The present invention relates to a spherical annular seal used in a spherical pipe joint for an automobile exhaust pipe.
Conventionally, as a spherical annular seal used in a spherical pipe joint for an automobile exhaust pipe, one disclosed in, for instance, U.S. Pat. No. 4,607,851 (hereafter referred to as the prior art publication I) is known.
The seal disclosed in this prior art publication I is heat resistant, excels in affinity with a mating member, and has remarkably improved impact strength, but has a drawback in that the seal often generates abnormal noise when it undergoes friction under dry frictional conditions.
The drawback of this seal is conceivably attributable to a large difference between the coefficient of static friction and the coefficient of dynamic friction of a heat resistant material, such as expanded graphite, for forming the seal, and to a phenomenon in which the seal formed of the heat resistant material shows, in the frictional resistance characteristic to the sliding velocity, negative resistance in which an increase in the sliding velocity results in a decline in the frictional resistance.
Accordingly, a seal which overcomes the above-described drawback has been proposed in Japanese Patent Unexamined Publication No. 24620/1983 (hereafter referred to as the prior art publication II).
This prior art publication II discloses a seal in which a heat resistant material formed of one kind selected from expanded graphite, mica, and asbestos or a mixture of two or more kinds selected therefrom is shaped with a reinforcing material formed of a metal wire net obtained by weaving or knitting fine metal wires, the surface of the seal being coated with a lubricating composition formed of polytetrafluoroethylene or a tetrafluoroethylene-hexafluoropropylene copolymer.
With this seal, the lubricating composition coated on the surface exhibits operational advantages such as a decrease in the coefficient of friction, the prevention of transfer of the heat resistant material forming a base onto the surface of the mating member, and a reduction of the difference between the coefficient of static friction and the coefficient of dynamic coefficient. In addition, since the frictional resistance of polytetrafluoroethylene relative to the sliding velocity does not show negative resistance, this sealing member has an advantage in that it is possible to suppress the generation of self-excited vibrations based on the "stick slip" in association with the above-described effects, thereby contributing to the prevention of the generation of abnormal noise.
Although the seal disclosed in the prior art publication II described above overcomes the drawback of the seal disclosed in the above-described prior art publication I in terms of the performance, the ambient temperature to which the seal disclosed in the prior art publication II is applicable is determined by the heat resistance of the lubricating composition coated on the surface. Hence, there is a problem in that the seal is inevitably restricted to use in the ambient temperature of not more than 300.degree. C. Additionally, the following new problem arises.
That is, there occurs a phenomenon in which, in a case where the seal is used by being incorporated in a spherical pipe joint for an automobile exhaust pipe, the lubricating composition coated on the surface of the seal becomes fused due to the action of heat from exhaust gases flowing through the exhaust pipe, and the fused lubricating composition fixedly adheres to the surface of the mating member when the exhaust pipe is cooled after the stopping of an engine, thereby hampering the relative angular displacement of the spherical pipe joint.
It has been experimentally confirmed that such a phenomenon occurs noticeably when the seal is used particularly under such a temperature condition of the exhaust pipe that the lubricating composition formed on the surface of the seal is fused, and when the seal is applied to a portion where a relative angular displacement applied to the pipe joint is small.
Accordingly, if such a phenomenon of fixed adhesion occurs, it is not only difficult to attain the original object of the pipe joint, but also there arises a problem in that, if a large angular displacement is applied to the pipe joint after the restarting of the engine, large abnormal noise is generated due to the overcoming of the phenomenon of fixed adhesion.